Picture on the left shows the Calibrated hydrophone just before deployment.
The Graphs: top: Spectrogram showing the sound energy levels at different frequencies that are detected when a ship passes close to the hydrophone; bottom: Try to avoid as there is a noisy gap.
Picture on the far right shows the mooring setup used for deployment. The sub-surface buoy keeps the cable taut so that the hydrophone is always an upright position.
Awareness about the hazard to marine life posed by underwater noise is steadily growing, as are monitoring efforts linked with such an environmental pressure. EU members, including Malta, have an obligation to acquire data under Descriptor 11 – ‘Introduction of energy, including underwater noise is at levels that do not adversely affect the marine environment’ - of the Marine Strategy Framework Directive (MSFD) which includes both impulsive and non-impulsive (continuous) sounds.
Impulsive sounds are typically brief, characterised by a large change in amplitude over a short time and containing a wide frequency range (commonly referred to as ‘broadband’). Such sounds are generated by explosions, airguns and pile-driving. Non-impulsive sounds can be broadband or more tonal, continuous or intermittent, with typically only small fluctuations in amplitude. Sources of non-impulsive sounds include shipping, construction (e.g. drilling and dredging) or wind farm operation.
Various anthropogenic activities which can result in the introduction of underwater noise in the marine environment, including (i) Shipping; (ii) Construction works; (iii) Windfarms; (iv) Sonars for bottom imaging and (v) Seismic sources (such as airguns). Given that Malta currently does not have the capacity to collect, on an operational basis, data on non-impulsive/continuous sounds and given the University of Malta’s shipping tracking infrastructure, a relevant research project was initiated by the Oceanography Malta Group (OMG) within the Department of Geosciences in 2019.
The research project in question consisted of the purchase and deployment at sea of an underwater noise meter (SoundTrap 3100 series) within the vicinity of a shipping hub (e.g. Grand Harbour. The meter is deployed close to the seabed at a depth exceeding 30m for 2-3 consecutive months before it is retrieved for data recovery and re-deployed, through an innovative permanent mooring design. The aim of the study, which is conducted by an MSc student (Ms. Marylyn Sammut) with the support of academic staff within the Oceanography Malta Group (Prof. Alan Deidun, Dr Adam Gauci), is to assess the impact that the shipping industry has on underwater noise levels by investigating correlations between the AIS (Automatic Identification System) database held in-house by the OMG within the Department of Geosciences and readings taken by the underwater noise meter.
The study, supported by funds from the University of Malta’s Research Committee and by the Interreg Italia-Malta BYTHOS project, also aims to characterise any seasonal trends in underwater noise variability - in so doing, the study will characterise the underwater soundscape of the marine area contiguous to a number of local shipping hubs, thus contributing to bridging the knowledge gap identified explicitly within the corresponding monitoring factsheet for the Marine Strategy Framework Directive’s Descriptor 11.